1. Field of the Invention
The present invention relates to a carrier, a developer, and an electrophotographic photoconductor for use in electrophotography, and to an electrophotographic image formation method capable of forming images on an electrophotographic photoconductor with a relatively low charging potential by the application of a low developing bias voltage thereto.
2. Discussion of Background
Since the invention of electrophotography by C. F. Carlson as disclosed as a Carlson process in U.S. Pat. No. 2,297,691, various improvements have been proposed based on the Carlson process.
Electrophotography, a representative example of which is the Carlson process, is currently widely used. The basic process of electrophotography comprises the steps of uniformly charging the surface of a photoconductor, selectively exposing the uniformly charged surface of the photoconductor to a light image, thereby forming a latent electrostatic image corresponding to the light image on the surface of the photoconductor, developing the latent electrostatic image to a visible toner image by a developer, transferring the toner image from the surface of the photoconductor to an image receiving medium, and fixing the toner image on the image receiving medium.
Conventional main methods for developing electrostatic latent images are a two-component development method using a two-component developer comprising a carrier and a toner, and a one-component jumping development method using a one-component developer comprising one component. For example, in the case of a conventional two-component developer comprising a toner and an electrically-insulating carrier such as a resin-coated ferrite, a carrier with an average particle size of about 80 .mu.m is used as the electrically-insulating carrier, and the concentration of the toner in the developer is set in the range from about 3 to 5 wt. %.
In the development step of the above-mentioned conventional development methods, a toner which is a colored resin powder is caused to be selectively deposited in a development area of the surface of a photoconductor in which latent electrostatic images to be developed are formed. On the other hand, in a non-development area of the surface of the photoconductor, the toner is caused not to be deposited. In order to carry out the above-mentioned development, it is required that the amount of magnetic particles contained in the toner be appropriately adjusted, or the surface potential of the photoconductor be set at about 500 V or more in the charging step, the potential difference between a high potential portion and a low potential portion in latent electrostatic images formed in the exposure step be set at about 400 V or more, and a developing bias electric field in the development step be set at about 500 V/mm or more. Furthermore, in this case, an electrical field of about 200 V/mm or more must be applied for elimination of the fogging from images is required in the development step.
To meet the above-mentioned requirements, a photoconductive material with a chargeability of about 500 V or more is required for the photoconductor. Therefore, there are considerable restrictions on the selection of a material for the photoconductor. In addition, since the charging potential required for the photoconductor is so high that the thickness of a photoconductive layer of the photoconductor has to be increased. For instance, in the case of an amorphous silicon (a-silicon) based photoconductive layer is employed, since the voltage resistance of the photoconductive layer is 12 V/.mu.m, the a-silicon based photoconductive layer must have a thickness of 34 .mu.m or more in order to charge the photoconductive layer to a charging potential of about 500 V or more. In the case of an organic photoconductor (OPC), an organic photoconductive layer thereof must have a thickness of 20 .mu.m or more.
In the case of an a-silicon based photoconductor, since an a-silicon based photoconductive layer thereof is generally fabricated by a plasma glow discharging method, the deposition rate of the photoconductive layer is so small that the manufacturing cost of the a-silicon based photoconductor and imperfections or defects thereof increase in proportion to the increase in the thickness of the a-silicon based photoconductive layer.
In the case of the organic photoconductor, the hardness of the photoconductive layer is so low that the thickness of the photoconductive layer is decreased by 1 .mu.m every about 10,000 copies when used in practice. As a matter of course, the chargeability of the organic photoconductor is gradually decreased while in use.
For example, in an organic photoconductor comprising an organic photoconductive layer with a thickness of 20 .mu.m at the initial stage, a charging defect is caused when not more than 50,000 copies are made. Namely, the organic photoconductor disadvantageously has an extremely short lifespan. It can be considered that this problem may be solved by increasing the thickness of the organic photoconductive layer to more than 20 .mu.m, for example to about 40 .mu.m. However, there is a limitation on the thickness of a film layer that can be made by a conventional coating film formation technique.
Furthermore, in order to charge the photoconductor to a high potential of about 500 V or more, a large-output-yielding charging unit and a corresponding long charging processing time are required. This brings about an increase in both the size of the charging unit and power consumption. In particular, in the case of an a-silicon based photoconductor, a large charging processing area is required because of its low charge-ability.
In the exposure step, there is required a light source with a sufficient amount of light emission for quick dissipation of electric charges from the surface of a photoconductor with a charging potential of about 500 V or more. Therefore, there are restrictions on the selection of a light source. Furthermore, the size of the exposure unit must be increased, so that high power consumption is required. In addition to the above, since high voltage is required for the developing bias voltage, the entire size of the apparatus has to be increased and accordingly the power consumption is inevitably increased.